Isolation, identification, and functional characterization of various leukocyte subpopulations have allowed us to correlate specific populations of cells with the pathophysiological phenomena in certain inflammatory and immunologically mediated diseases. Studies of the specific effects of immunosuppressive agents such as corticosteroids have delineated the differential sensitivity of these subpopulations of cells to therapeutic intervention. In extension of previous studies in this area, we have further demonstrated the differential effects of these agents on distinct subpopulations of mononuclear cells mediating cytotoxic killer cell function against various tissue targets. Such studies have clearcut clinical relevance in inflammatory and autoimmune diseases, organ transplantation, and tumor surveillance. We have demonstrated the presence of cytotoxic effector cells in human bone marrow, thus adding to our understanding of possible mechanisms of the graft versus host reaction seen in bone marrow transplantation. We have recently developed a highly reproducible hemolysis-in-gel plaque forming cell assay for the measurement of single cell antibody production following primary in vitro activation of human B lymphocytes from tonsil, bone marrow, spleen and blood. Such a model will allow in depth probing of the mechanisms of activation and suppression of human B lymphocytes towards antibody production.